/**
 * Shadow-DOM-safe containment check.
 *
 * `Node.contains` does not pierce shadow-root boundaries: a node inside a
 * shadow tree will report `parent.contains(child) === false` even when the
 * shadow host is itself a descendant of `parent`. This helper first tries
 * the native method (fast path) and, if the child is inside a shadow root,
 * walks back up through shadow-host boundaries to check containment.
 */
export declare function contains(parent?: Element | null, child?: Element | null): boolean;
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